The present invention relates to a roller magnetically compensated for deflection. More particularly, the invention relates to an end-loaded roller such as a press roller or calender roller of a paper machine. The roller has a non-rotating axle having spaced opposite ends at which said roller is loaded and a cylindrical shell rotatably mounted on the axle. The axle has a magnetic core portion within the shell. The magnetic core portion has pole shoes spaced from the shell by air gaps and magnetizing coils thereon at corresponding ones of the pole shoes. The coils produce a magnetomotive force and are positioned in a manner whereby magnetic flux passing through the air gaps between the pole shoes and the shell creates a compensating force field between the shell and the core.
U.S. Pat. No. 3,456,582 discloses an electromagnetic press nip defined by two rollers, wherein the nip pressure is produced by a roller having permanent magnets inside it and a roller of smaller diameter which bends with comparative ease. The magnetic flux path is closed substantially axially in these rollers through the roller with the smaller diameter. The contact line between these rollers functions as the air gap. Magnetic forces develop at the contact line between the rollers and the web passes between said rollers at said contact line.
A roller of the aforedescribed type is also disclosed in "Wochenblatt fur Papierfabrikation, Biberach Marz 1978. Nr. 5, p. 194 (Baumgarten, H. L.: Uber die thermo-mechanische Oberflachenbehandlung und Kalibrierung von Papier und Karton)". The rollers disclosed in these references are not loaded at their ends and the nip pressure is not produced by the loading forces, but merely by magnetic forces.
The roller of the invention, when defining a nip together with an opposing roller, is intended for the pressing treatment of web-shaped materials. It is commonly understood that when such rollers are loaded at their ends they undergo such deflection that the line pressure is higher in the marginal parts of the press or calender nip than in the central area. Attempts made in an effort to avoid this drawback include cambering of the rollers, which is making the roller shell barrel-shaped, beforehand. However, conventional cambered rollers permit the achievement of uniform line pressure at one specific load, only. "Camber" rollers have been developed in order to eliminate this drawback.
"Camber" rollers are exemplified, for example, by the "Kusters" roller, disclosed in U.S. Pat. No. 2,908,964. In the "Kusters" roller, a chamber containing fluid under pressure is provided between the stationary roller axle and the rotary roller shell at a given sector. Regulation of the pressure of the fluid in the chamber permits the deflection of the roller to be compensated. The drawbacks of the Kusters roller are sealing problems and pressurized fluid leakage resulting from such problems. Another drawback of the Kusters roller is its sluggish response to pressure changes. The speed of ascent is about 30 seconds and the discharge speed is about 10 seconds. As a result, considerable quantities of paper which must be rejected may be produced in a paper machine, due to pressure changes, for example.
Deflection-compensated "CC" rollers, of the Beloit Co., are also known in the art. CC rollers have pressing shoes which rub against the inner surface of the roller shell. The CC roller, similarly to the Kusters roller, has the drawbacks of pressure fluid sealing problems. The CC roller also has the drawback of comparative difficulty in providing adjustments which are asymmetrical in the axial direction of the roller.
In the rollers heretofore mentioned, the compensating forces are produced by pressure chambers of friction shoes, which require a high degree of smoothness of the inner surface of the roller shell. This involves higher roller manufacturing costs.
Another drawback of the aforementioned rollers of the prior art is the increased need for power to rotate the roller, resulting from the sealing elements and other friction elements.
Press rollers acting in accordance with electromagnetic forces are known in the art. An object of the invention is to further develop exactly this type of roller. Reference may be made in this connection to U.S. Pat. No. 3,456,582. This patent discloses a roller which differs from the roller of the invention in the design and implementation of details and also that the roller of this patent is not loaded at its ends.
Regarding the state of art associated with the invention, reference is made to German printed publication DT-OS No. 1,761,641. This publication discloses a roller which is deflection-compensated by permanent magnets mounted both on the stationary axle and the rotating shell. The compensating forces are controlled by axial adjustment of the roller axle and roller shell with reference to each other, thereby changing the mutual positioning of said shell and axle and thus achieving control of the compensating forces.
Regarding the state of art associated with the invention, reference is also made to U.S. Pat. No. 3,489,079, which is the equivalent of Finnish patent application No. 2283/67. The roller disclosed in this patent has magnetic shoes disposed in "V" configuration and producing attractive forces at the air gaps between the two poles of said magnetic shoes and the inner shell of said roller.
Reference is made to Finnish Pat. Nos. 52 394 and 58 662 of the present inventor. These patents are equivalent to U.S. Pat. Nos. 4,062,097 and 4,301,582, respectively. The roller of Finnish Pat. No. 58 662 has a magnetic core with three pole shoes in its cross-section. The magnetic core has magnetizing coils which create a magnetomotive force, so that the magnetic flux producing the actual compensating force field passes through the centermost pole shoe of said core and is closed back to said core through the roller shell and two substantially opposed pole shoes of said core.
The general object of the invention is to further develop the aforementioned rollers and, more specifically, the magnetic roller disclosed in Finnish Pat. No. 58 662. The specific objects of Finnish Pat. Nos. 52 394 and 58 662 was to provide a roller with faster control characteristics than previously, compared to the Kusters and CC rollers. An additional object of these Finnish patents was to provide a roller of lower cost which operates with lower drive power, compared to the aforementioned mechanically and hydraulically compensated rollers. This is accomplished due to the fact that no particular smoothness requirements are imposed on the inner surface of the roller shell because there is no direct contact and no need for sealing.
When a rotating shell roller is used, for example, as disclosed by Finnish Pat. No. 58 662, said shell consists of a cylindrical part of continuous material such as, for example, of cast steel, through which the magnetic fluxes between the poles of the stationary magnetic core are closed, and the rotation of said shell produces considerable eddy current losses, even if the magnetizing current is direct current. It is well known, of course, that the eddy current losses increase with the square of the frequency. In this case, the eddy current losses increase with the square of the speed of rotation. Even if the magnetizing current is direct current, a flux is produced in the shell which changes at different points, due to its rotation.